This invention relates to a rotary head type magnetic recording/reproducing apparatus (which will be referred to as a VTR hereinafter, and which includes such apparatus having no recording function), and more particularly to such apparatus constructred so as to make it possible to automatically bring the tracking state to an optimum tracking state even upon slow motion or still picture reproduction.
Conventionally, the rotary heads in a VTR are required to trace on recorded tracks with high fidelity even upon slow motion or still picture reproduction. So, the tracking controlling is conventionally performed by: preliminarily recording control signals corresponding to the rotary positions of the rotary heads; and controlling, upon reproduction, the relative positions between the rotary heads and the tape on the basis of the control signals so as to cause the rotary heads to trace on the same tracks as those used for recording.
In this case, the tracking controlling is performed at only the starting point or ending point of each recorded track. So, the tracking, on the way, of recorded signals which might contain important information would undesiredly become insufficient. Accordingly, a conventional VTR has a manual tracking shifter which is operated by an operator for setting the tracking condition at an optimum S/N ratio of reproduced pictures. Such an operation has problems in that such an operation is very troublesome to the operator since it is difficult for the operator to judge the optimum S/N ratio.
For solving these problems, a VTR was developed recently, which does not require the use of a tracking shifter, and in which the rotary heads continuously scan the recorded tracks correctly from the starting point to the ending point of the recorded tracks. This operation principle is that the rotary heads are held via a piezoelectric element which displaces deflectingly in the lateral direction of the recorded tracks, and the rotary heads are moved to keep the on-tracking by moving the piezoelectric element (positionable element). In this case, control signals to represent direction and amount of the deviations of the rotary heads from the recorded tracks are necessary. Such control signals are obtained by: vibrating the rotary heads by a reference frequency f.sub.c of a sinusoidal wave signal (this vibration is called wobbling); and synchronously detecting the then generated envelope detection output. This performance will be described in more detail with reference to FIG. 1. FIG. 1 shows relationship between recorded tracks T and the scanning traces R of the rotary heads, and the envelope detection output waveforms of the RF signals then obtained from the rotary heads.
In FIG. 1, A1 shows the case when the vibration center of a rotary head passes the center position of the recorded track T, wherein the only portion of the recorded track actually reproduced by the rotary head is shown therein by hatching on the recorded track T. The envelope waveform of the reproduced signal then is a sinusoidal wave of the frequency 2f.sub.c as shown by A2. The case B1 represents the case when the vibration center of the rotary head has displaced upward, wherein the full line shows the state when the rotary head has displaced as much as the wobbling amount, while the broken line shows the state when the rotary head has displaced by an amount smaller than the wobbling amount. The envelope waveform in this case is a sinusoidal wave of a reference frequency f.sub.c of the wobbling, and an opposite phase to the reference frequency signal, as shown by B2. The amplitude of the envelope waveform is proportional to the displacement amount, i.e. track deviation or tracking error amount. Likewise, C1 of FIG. 1 represents the case when the vibration center of the rotary head has displaced downward. Then, the envelope waveform C2 is a sinusoidal wave of the frequency f.sub.c and is in-phase with the reference frequency signal. It is apparent from the above descriptions that, paying attention to reference frequency component (wobbling frequency f.sub.c) of the envelope waveform reproduced from the rotary head, its phase represents the direction of track deviation or tracking error, and its amplitude represents the amount of track deviation or tracking error. Therefore, by using this signal as a control signal, a servo system can be constructed for causing the rotary head to keep on-tracking on the recorded track T.
However, a conventional automatic tracking system to operate on the above described principle have the following drawbacks. That is, the track deviation amount or tracking error amount correctable by the conventional automatic tracking system is limited by the wobbling amount. The case D1 in FIG. 1 shows the case when the vibration center of the rotary head has displaced upward by an amount exceeding the wobbling amount. In this case, the total level of the envelope is lower than that in the case B1, but the reference frequency component in this case has exactly the same emplitude as that of the case B2. That is, when there is occurring a tracking error exceeding the wobbling amount, the amplitude of the reference frequency component of an envelope detection output is not totally in proportion to the tracking error, but its amplitude becomes saturated or levels off by the wobbling amount. This is true also for the case when the vibration center of the rotary head has displaced downward.
It is apparent from the above-described relationships that the characteristics of the control voltage (obtained by synchronously detecting the envelope detection output by the reference signal) relative to the tracking errors can be shown by FIG. 2. The horizontal axis of FIG. 2 represents the tracking error, while the vertical axis represents the control voltage detected from the envelope. In the range of the tracking error between -T.sub.W (upward T.sub.W error) and T.sub.W (downward T.sub.W error), the control voltage obtained is proportional to the tracking error, whereas in the range of errors greater than T.sub.W, the control voltage is constant, and the control voltage vanishes at T.sub.D. The value T.sub.D represents a critical point where the head-scanning trace is tangent to the recorded track without any overlapping between the trace and the track. If a tracking servo is controlled by using such a control voltage, on-tracking can be attained in the tracking error range of -T.sub.W to T.sub.W by realizing the servo mechanism, whereas ontracking cannot be attained but the head is only shifted by a constant amount to the center of the track in the tracking error range exceeding .+-.T.sub.W. In other words, the controllable range is limited to the range between -T.sub.W and T.sub.W.
As apparent from the foregoing descriptions, the range -T.sub.W to T.sub.W corresponds to the wobbling amount. Therefore, according to such conventional method, the wobbling amount is required to be increased in order to widen the controllable range of tracking, which, however, causes an increase of jitter, chroma color shading, etc., whereby the reproduced picture quality is very much deteriorated. On the other hand, if the wobbling amount is reduced, the controllable range of tracking is also reduced, which causes generation of a noise band in a reproduced picture.